1. Field of the Invention:
The invention relates to electrical apparatus and, more particularly, to current limiting fusible devices.
2. Description of the Prior Art:
Fuse-protected capacitors are widely used in the transmission and distribution of electrical energy to provide power factor correction. Typical applications range from small single-capacitor installations to giant central generating station facilities having many banks of multiple capacitors. The high voltage stress placed upon these capacitors can occasionally cause break-down of the capacitor insulation, resulting in a short circuit failure through the capacitor. If adequate protection is not provided, the capacitor case may then rupture and explode. Even when the individual capacitors are protected by fuses, the tremendous energy stored in parallel-connected capacitors will surge through the fuse of a failed capacitor causing the fuse to operate with a prominent audible and visual display attended by production of large volumes of ionized gases. This can then result in arcing to other installation structures, especially on indoor capacitor installations.
Where a large number of capacitors are present in a single installation, fault current resulting from the surge of energy from the capacitor bank through the failed capacitor has a very fast rise time in comparison with a normal 50 or 60 cycle current rise time. In other words, the fault current resulting from the dumping of capacitive energy through the failed capacitor has very high frequency components.
On smaller capacitor systems, on the other hand, the major fault current through a failed capacitor is likely to be 50 or 60 cycle current flowing from the line, rather than capacitive stored energy current. Protection of capacitors against the two types of fault currents to which they are susceptible requires different fault clearing characteristics. It is therefore desirable to provide a fuse having the capability to protect electrical systems from damage due to failed capacitors caused by both the high frequency capacitive fault current and the standard power line frequency fault currents.
In copending U.S. Pat. application Ser. No. 730,097, filed Oct. 6, 1976, by J. N. Santilli, there is disclosed a two-section fusible device having a current limiting portion to provide protection against high fault currents and an expulsion-type section to provide low current fault protection. It is desirable to provide an improved fusible device having a smaller visual and audible display upon operation. In addition, it is desirable to provide a more rugged fusible device in a smaller case which is particularly adapted to stand up to temperature cycling produced, for example, by high in-rush current.
Since proper fuse operation can result in a short circuited capacitor sustaining no visible damage, it is desirable that the fuse provide an indication of its operation so that periodic inspection of capacitor installations by maintenance personnel will result in the discovery of the failed unit. The large volume of hot ionized gases produced during operation of some prior art fuses has resulted in the need for mufflers or condensers to provide protection against arcing or flashover during fuse operation on an indoor or enclosed capacitor installation. These mufflers and condensers make it difficult to determine whether or not a fuse has operated. It is therefore desirable to provide a fusible device which minimizes the expulsion to the environment of hot ionized gases, thereby eliminating the need for mufflers or condensers and providing a more positive indication of operation.